What is the purpose of IWindowCreator?

[Ghostbird]

I recently upgraded my MAUI 8 app to MAUI 9. One of the things I had to change was that I had to replace the Application.MainPage = with an override of Application.CreateWindow.

So I overrode that function with the logical implementation:

  protected override Window CreateWindow(IActivationState? activationState)
  {
    ArgumentNullException.ThrowIfNull(activationState);
    var window = base.CreateWindow(activationState);
    window.Page = activationState.Context.Services.GetRequiredService<AppShell>();
    return window;
  }

However, to facilitate this, I must also register an implementation of IWindowCreator with a trivial implementation.

public class WindowCreator : IWindowCreator
{
	public Window CreateWindow(Application app, IActivationState? activationState) => new();
}

This works perfectly, but it leaves me with questions.

1. Why is no mention made that you must register an implementation of IWindowCreator?
2. Why is no default implementation of IWindowCreator provided?
3. Or if no default implementation of IWindowCreator is meant to be provided, why does the CreateWindow method not fall back to the trivial implementation: return new()?

The current state of the code seems incomplete, and ready for further development. I saw StackOverflow answers that override CreateWindow without calling the base class, or with an implementation that first creates a window and adds it to Windows. In doing so, you respectively; discard all the built-in guards, or create a window that will immediately be discarded once IWindowCreator is registered properly in DI. The current state of the MAUI code will lead naive programmers to write code like that, and suggest it on StackOverflow. I expect MAUI to later complete this incomplete mechanism, and that will cause lots of failures in such currently suggested implementations of CreateWindow.

[MartyIX]

I can see that https://learn.microsoft.com/en-us/dotnet/maui/fundamentals/windows?view=net-maui-9.0#decouple-window-management-from-the-app-class mentions that one must register the creator:

builder.Services.AddSingleton<IWindowCreator, WindowCreator>();

Perhaps you can file an issue here https://github.com/dotnet/docs-maui/issues to ask for more details or create a PR to add information that is not obvious.

[Ghostbird]

Thanks for pointing that out. I get that it's for decoupling, and I'm very happy with that. Creating a docs issue might be a good idea. From a point of decoupling, I'm mostly unclear whether this is an IWindowCreator or an IMainPageWindowCreator the latter is what the documentation suggest.

In my current code I've used it to decouple the Window creation from the app using the IWindowCreator. However, given the name, I couldn't decouple the main page creation from the app. After all, IWindowCreator suggests that it will be used to create every window, not just the window that shows the main page.

[MartyIX]

From a point of decoupling, I'm mostly unclear whether this is an IWindowCreator or an IMainPageWindowCreator the latter is what the documentation suggest.

I think that it's simply IWindowCreator and not IMainPageWindowCreator. The naming is IMHO as follows:

• For window think of a Windows window.
• For page think of some content in an actual operating system window.

IWindowCreator should be called for all windows you create but not for all pages you create.

I recently upgraded my MAUI 8 app to MAUI 9. One of the things I had to change was that I had to replace the Application.MainPage = with an override of Application.CreateWindow.

But the deprecation warning recommends something like:

Application.Windows[0].MainPage = something

doesn't it? Is it not possible in your use case?

[Ghostbird]

I'm currently using this approach where I register the class below as a singleton IWindowCreator.
I improved upon some problems I recognised in the documented approach:

• Decouples main page lifetime from the application and its main window entirely. E.g. it can be called to recreate a main window without recreating the main page.
  Think of apps like Spotify and Discord that close their main window, but keep running, and leave some method to create a new main window again, that shows the page as it were when you closed it.
• Can be called to create subsequent non-main windows that will not show the main page.

public class WindowCreator : IWindowCreator
{
  public Window CreateWindow(Application app, IActivationState? activationState)
  {
    var window = new Window();
    if (!app.Windows.Any())
    {
      // First window created shows the main page.
      // Note: Your main page class will most likely be different.
      window.Page = activationState?.Context.Services.GetRequiredService<AppShell>();
    }
    return window;
  }
}

If using this approach, do not set Application.MainPage, nor override Application.CreateWindow.